The issue is that the index pattern (mediaserver_2017_2017-10*) matches the destination rollup index. So when the job is rolling up data, it accidentally searches the destination rollup index too, which has different field names, and so it throws an exception.

That bug is fixed in 6.4, we throw an exception when you try to create the job telling you that the pattern would match itself.

The reason it worked when you switched from hyphen to underscore is likely because the pattern no longer matched. Sorry for the bug, it's definitely irritating!